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Thread: RPM and bullet stability?

  1. #1

    Default RPM and bullet stability?

    I have found a accurate load for my CZ527 22 hornet using the Berger 40gr HPV bullet. It state on the box 1:15" twist rate or better. At a averge velocity of 2995fps I get very tight five shot clover leaf groups say 3/8" at 50 yards and thats out side to out side. Increasing the the charge from 12.6 to 12.7grs drops the velocity to 2915 fps and opens the groups to 5/8".

    Does the higher RPM caused by the higher velocity help make this bullet more stable or is it just in the rifles sweet spot for accuracy?

    As a follow up the accuracy loads are (12.5grs AA1680, Federal 205, Rem Brass, Sierra 40gr SP, @ 2728fps) and (12.6grs Lilgun, CCI 500, Win Brass, Berger 40gr HPV @ 2995fps). Both produce 5 shot groups of .250" CC or better @ 50 yards.

    Now that I am confident in these loads I will load up 50 rounds of each and more to a rifle range for some 100 & 200 yards testing.

  2. #2
    Join Date
    Jan 2005


    An interesting subject, I have no answer to your question but I have lots of other info about rpm, etc.

    The stability requirement for any bullet is based on it's length, longer needs more spin but it isn't rpm necessarilly, it is surface feet per minute. And the length is not simply a longer bullet but the length campared the diameter. Specifically the length of a bullet in the Greenhill formula is in calibers. We can convert it to inches or centimeters but the real requirement for a bullet is based on its' length in calibers. Also faster velocity will impart a higher RPM which would increase the surface-feet-per minute so sort of the same thing.

    Now back to the Hornet, First the increase of .1 grain of powder even in the dimutive Hornet case is so small that any velocity gain will be well within the SD predicted velocity spread (ES) for the next lower charge weight (12.6 grs) What this means is that if you fired 10 rounds of 12.6 grs then 10 rounds of 12.7 grs. Both mean velocities would fall within the expected ES of the other. So you can't prove anything with that.

    Secondly, the slight difference in velocity will not effect the stability of the bullet to any measurable degree. It could and likely will effect the accuracy because it changed the hummer point of the barrel. It's sweet spot, so to speak. Also, resting deep in this equation lies the human factor and that is also the best answer to the difference in accuracy. Your shooting ability was worse for the 5/8" group. If you had shot 10 five shot groups with each, all going through the chronograph window, we would have much better, more usable data to support or cast asunder any theory about velocity/rpm/twist rate/shooting ability that you may want to ply.
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?

  3. #3



    I was wondering if my 1:16 twist was the issue.

    I guess I am lucky in that the 12.6 is both 70 fps faster and that the higher velcity of 2995fps is the most accurate load. From 12.0 to 12.6 grs there is not much of a velocity difference say 30fps tops. At the 12.7grs I figure i just have to much compression and so the velocity fall off. This is as acurate as the shorter Sierra 40gr SP using AA1680/fed205 @ 2728fps. In any case it still MOA and better if I keep the loads in its sweet spot.
    And the spent primers are very round at the edge and not much flatting going on there. So I hope case life will be good. I am using Lee's Collet die to just size the neck.

    The kicker was following Speers advise and using the CCI 500 small pistol primers the fed205's where 3100-3200fps and grouped like 1.5" @ 50 yards. If you remember I followed Hodgdons advise and up my starting load from 11.0grs (3100fps)to 12.0grs (3200fps) lucky I didn't split a case.


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